This invention relates to a method of detecting an indicating device in a restricted zone of investigation and to a system for carrying out the method.
The invention is intended to be used primarily for goods monitoring in the trade, especially in the everyday commodities trade, but can be used also for indication of other kind, for example for indicating persons.
In Swedish patent application No. 8400826-7, a system for detecting the presence of indicating devices is described which have the form, for example, of a narrow and thin, but relatively long strip of a highly permeable material, such as being commercially available under the name Permalloy. The presence of such strips is detected by means of coils, which emit and receive magnetic fields, and by means of devices for the detection of signals received.
When such a strip is exposed to a relatively weak external magnetic field, it assumes in its longitudinal direction a magnetic flux density, the magnitude of which can be twenty thousand times higher than in the external magnetic field. The presence of such strips is detected by a detector device, in which variations in voltage induced in a receiver coil are detected.
According to said patent, two coils are caused to emit a magnetic alternating field, where a first one of the coils emits an alternating field with a frequency (f.sub.1), which is different from the frequency (f.sub.2) of the field emitted by the second one of the coils. Difference and sum frequencies n.multidot.f.sub.1 +m.multidot.f.sub.2 arising by intermodulation by the indicating device, where n and m are positive or negative integers, are received by one or several coils, whereafter the signal thus received is signal processed in a detector device for determining whether or not there is intermodulation, i.e. whether there is or is not an indicating device in the monitoring zone. The said intermodulation is caused by non-linear magnetic properties of the indicating device.
In practice, however, even objects made of iron, for example trolleys, tins etc., which normally are present in the investigation zone, give rise to intermodulation. Interferences by other magnetic fields and iron objects also can result in, that signals received at the detection are interpreted as the detection of an indicating device.
A tin, for example, located close by a coil emitting a high-frequency signal, by intermodulation can give rise to a signal received, the characteristics of which correspond to a signal from a strip of highly permeable material located for away from said coil.
For this reason it is necessary, for example, that intermodulation products of different order must be detected in order to distinguish if a signal received originated from an indicating device in the form of a strip or from another object in the investigation zone.
It is a desire of extreme importance to prevent such monitoring systems from sounding alarm caused by objects other than special indicating devices.